1. Field of the Invention
This invention relates to force measuring equipment, such as weighing scales, employing at least one load cell unit for sensing a force to be measured and acting as a mechanical-to-electrical transducer to provide an electrical output whose characteristics are correlated in predetermined fashion with the magnitude of the mechanical force being sensed by the unit.
More particularly, this invention is concerned with improving the accuracy of such force measurements by substantially eliminating or significantly minimizing the side loads typically transferred to the load cell unit along with the primary component of the force to be measured.
2. Specific Background Considerations
In order to understand the real nature of the problem addressed by this invention and the full significance of the manner in which it is solved by the invention, as well as the ultimate importance of certain differences between the integrated and successful approach utilized by the invention and the partially similar but incomplete and inadequate techniques employed in prior art devices now in common usage and heretofore accepted as representing the best available "compromises", it is necessary to recall certain basic aspects and to consider certain more detailed characteristics of large weighing scales (and similar force measuring equipment), load cell units of the "compression" type typically employed in such equipment, and flexure plates of the type sometimes utilized in conjunction with such units.
Typical large weighing scales have extensive and massive weighbridge structures, usually formed at least in part of steel structural components, receiving thereon the load to be weighed. Portions of the weighbridge structure are supported upon the upper ends of a number of "compressible" load cell units (or, as in this invention, assemblies incorporating such units), and the lower ends of such units (or assemblies) in turn rest upon fixed support structures typically mounted on the ground. The presence of a load to be weighed upon the weighbridge structure urges the latter downwardly with increased force upon the load cell units, which are thereby increasingly "compressed" and provide electrical outputs from which the weight of the load may be determined and indicated in known manner. Manifestly, it is only the vertically downwardly directed force component of the weight of the load which is relevant to the desired measurement.
However, the vertically downwardly directed force component of the load weight must be transferred to the load cell units via the weighbridge structure, which, although typically restrained against gross lateral or rotational movement relative to its support structure (by check rods or the like), is susceptible to the effects of spurious factors capable of causing secondary lateral and rotational movements of the portions thereof that transfer the primary vertical force component of the weight being measured to the load cell units. Such lateral and rotational movements of the weighbridge structure are inherent in any practical scale construction, and typically so to at least an extent such that, in the absence of effective efforts to negate their effects, the resulting "side loads" or lateral and rotational force components presented by the weighbridge structure to the load cell units (or assemblies incorporating the latter) are of significant levels sufficiently in excess of acceptable tolerances to adversely affect the accuracy of the desired weight measurement or, in extreme cases, even damage the load cell units.
The load cell units typically involve a base element resting upon the underlying support structure and an ideally vertically upwardly extending shiftable element upon which the weighbridge structure is resting, so that the axis of "force measuring compressibility" of the unit will be properly oriented for sensing of the primary vertically downwardly directed force component of the load carried by the weighbridge structure. Although the objectionable lateral and/or rotational movements of the weighbridge structure relative to the underlying fixed support structure involve linear or horizontal angular displacements, they also result in application of the force of load to be measured upon the load cell unit along a vector tilted away from its ideal vertical orientation. Accordingly, it is convenient and customary to make quantitative references to the side loads applied to a load cell unit in terms of the angle away from vertical of such force or load vector applied to the load cell unit caused by the side loads involved. In those terms, a typical load cell unit will begin to suffer serious deterioration of its force measuring accuracy whenever the tilt of such vector induced by side loads exceeds about 0.5 degrees from vertical.
It should be noted that such impairment of accuracy results mainly from the peculiar sensitivity of load cell units to even relatively small magnitudes of side load force components.
Among the spurious factors which commonly cause side loads are imperfections of structural alignment of parts of the scale or other force measuring equipment during fabrication or installation thereof, thermal expansion or contraction of structural parts when the temperature departs in either direction from that for which the juxtapositions of parts were adjusted during installation, and deflections of parts induced by the load being weighed or other force being measured. Initial imperfections of alignment of structural parts, such as a non-horizontal girder flange or an out-of-level support surface on the portion of the fixed base structure underlying a load cell unit, can produce "build-in" rotational tilts in the axes of the load cell units. In carefully constructed large scales, the tilting of the axes of the load cell units resulting from imperfection of initial structural misalignments can normally be held to somewhat less than 0.5 degree; however, errors in fabrication or installation can cause much larger "built-in" tilt angles of the axes of one or more load cell units of a particular large scale ranging up to 3.0 degrees or more, tilt angles from such cause commonly exceeding 0.5 degrees as a practical matter in too many instances. Since large scales are often located out of doors and subject to widely varying ambient temperatures, the side load tilting of the axes of the load cell units or the vector force applied thereto from ambient temperature changes may typically range up to as much as 1.0 degree. Side loading of the load cell units resulting from load induced deflection of structural parts typically may be of the order of 0.5 degree depending, of course, upon the over-all construction employed. Since such tilt angles of the axes of the load cell units or the force vector applied thereto from side load effects may act in various directions and may be additive under particular circumstances, it is apparent that such tilting to and beyond the point at which serious deterioration of the accuracy of measurement commences are common in practical situations and must be somehow avoided or the effects thereof negated if accuracy of measurement is to be preserved.
To that end, efforts have been made to employ flexure plates for the transmission of force from the weighbridge structure to the load cell units, such devices essentially comprising a steel plate having a pair of opposite marginal sections thereof rigidly mounted, with the intervening span of the plate being adapted to flex in the direction of its thickness in response to a force applied near the center of such span. Such flexure plates are helpful and effective in largely eliminating side loading due entirely to linear type horizontal shifting of the weighbridge structure, since the flexure plate can normally merely flex in the direction of its thickness but not move laterally thereto because of the rigid securement of its marginal sections and the inherent resistance of plates to distortion in response to force components acting along their major plane; however, unfortunately, a flexure plate becomes almost totally ineffective for eliminating side load effects when it is subjected to rotational as well as lateral side loading forces, which is frequently the case with the side load problem experienced with weighbridge structures and the like.